The present invention relates to sensing the pressure and temperature at which a heated fluid changes from a single phase (liquid) to a 2-phase (liquid-plus-vapor) condition.
Although the invention has general application to detection of the temperature and pressure at which boiling or flashing occurs in a heated fluid, for concreteness of description, an illustrative example is described in which the flash pressure, or pressure and temperature at which steam and/or gases are evolved, is detected in a geothermal liquid.
Geothermal liquids under high pressure in their native rocks generally contain dissolved substances that become gaseous at low pressure, such as CO.sub.2, N.sub.2, CH.sub.4, and many others. The relative proportions and the total amounts of the dissolved gases present depend on the history of the geothermal water.
Exploiting the geothermal water generally involves reducing the total pressures so that dissolved gases escape. If the waer is superheated prior to pressure reduction, some of the water will "flash" to steam and the gases referred to above will mainly follow the steam phase. Since there are several important chemical effects associated with the gas components, it is important to know their concentrations in the geothermal liquid.
One useful indicator of gas concentration is the collective pressure of all the gases as they occur in the unflashed liquid. The collective pressure is the sum of the partial pressures due to individual gas components. The concentration of a single species of gas in the liquid is proportional to its partial pressure in the mixture, but different gases have different proportionality constants. Since the partial pressure of one gas acts independently of the partial pressures of the other gases, the several partial pressures are additive in the mixture. Furthermore, they provide an increment of pressure that exists over and above the vapor pressure of the water of the geothermal fluid. Because of this, a geothermal fluid tends to boil (flash) at a higher pressure than one expects either for pure water at the same temperature or for water dosed with dissolved solids. The latter is the case for geothermal fluids.
This effect has been the basis for an apparatus described by G. D. McDowell in Geothermics, v. 3, p. 100, 1974 and by A. J. Ellis and W. A. J. Mahon in Chemistry and Geothermal Systems, Academic Press. The described apparatus measures the non-water gas pressures of a geothermal fluid by placing a small chamber of pure H.sub.2 O inside a larger chamber in which a mixture of geothermal liquid and vapor achieves thermal equilibrium with the H.sub.2 O in the smaller chamber. A differential manometer is used to measure the pressure difference between the chambers. Mathematical equations and certain assumptions convert the measured pressure difference into a concentration of gases.
The method is useful for geothermal fluids which contain more than 1 wt % of gas. The analytical sensitivity is relatively low because the method requires that the boiling (flashing of the geothermal fluid) be fairly advanced in the larger chamber mentioned above. When more water flashes, the other gases are diluted and their pressures are smaller and therefore harder to measure accurately.
U.S. Pat. No. 3,264,863 teaches the use of acoustic input to a fluid to detect cavitation noise indicative of incipient boiling. This apparatus electronically measures the acoustic energy which must be injected into the fluid to produce cavitation noise as the fluid temperature and pressure approach the point at which steam can evolve. The requirement for acoustic input as well as electronic measurement of cavitation noise results in a complex and expensive system.